Conventionally, as a flowmeter of this kind, for example, an impeller-type flowmeter described in the following Patent Document 1 is known. The flowmeter is for detecting a rotation of an impeller with a magnetic sensor, and between a measuring chamber in which the impeller is disposed and the magnetic sensor, a heat insulating plate made of synthetic resin is arranged and a heat insulating air chamber is formed. Also, at the portion constituting the outer peripheral wall of the heat insulating air chamber, a heat radiating vent hole which allows air to come in and out between the heat radiation air chamber and the outside and a heat radiation fin are provided (see FIG. 1 of Patent Document 1).